There is a long felt need for a single chemical that can efficiently impart both oil and water sizing properties to paper, since many grades of paper require both oil resistance and water resistance. Commercially available chemicals that are said to provide both oil and water sizing properties are not in fact efficient water sizing agents. Consequently, when both oil and water sizing properties are required, the conventional practice is to use an oil sizing agent and a separate water sizing agent. The effect is to reduce the efficiency of both the oil sizing agent and the water sizing agent.
Many end uses of paper products require that these cellulosic materials be resistant to penetration by various liquids ranging from water to hot oil and grease. Although many known chemicals act as efficient oil or water sizing agents for paper, few of them act as both water and oil sizing agents, and none of those provide efficient sizing against penetration by water, hot water, oil, and hot-oil. Reference is made in this connection to Kirk Othmer Encyclopedia of Chem. Tech., 3d ed, v 16, p. 808-814, 1981; "The Sizing of Paper", 2nd Ed., W. F. Reynolds Editor, Tappi Press, 1989.
Examples of end use products requiring some combination of water, hot water, oil or hot oil resistance include textile products, leather goods, reprographic paper, food board containers, wax coated paper, labels, coupons, food wrap, pet food containers, nonwoven medical wear, candy wraps, food containers for use in microwave ovens, and molded food containers.
Commercial papermaking conditions require that any chemical additive, such as a sizing agent, must be either readily water dispersible in its neat form, or easily convertible into a water dispersible emulsion or stable aqueous dispersion.
There are two physical forms of sizing agents in aqueous delivery systems: monomolecularly dispersed ionic salts and dispersions or emulsions of particulates or oils. Sizing agents in particulate form must be low-melting solids or liquids so the material will spread throughout the paper when heated in the dryer section of a paper machine.
When the sizing agent is added to the pulp slurry (internal addition) it must adhere to the pulp or it will not be retained in the paper. This may be accomplished by means of opposite charge attractions between the pulp and the size and by physical trapping of particles of size during web formation.
For efficient oil and water sizing, both hydrophobic and oleophobic constituents of the size molecules must be oriented away from the paper and hydrophilic constituents oriented towards the paper, so that liquid penetrants are exposed to a low surface energy hydrophobic and oleophobic surface. The sizing agent must maintain that molecular orientation to function efficiently. This orientation can be maintained by opposite charge bonding, ionic bonding, or by covalent bonding between the size and the pulp. Covalent bonding is the strongest form of bonding. Weaker opposite charge and ionic bonds are more susceptible to cleavage by aqueous liquid penetrants, especially when the penetrants are hot.
Efficient commercial oil sizing agents for paper also contain hydrophilic functional groups, such as phosphate, carboxylate, or chromium salts, that maintain water dispersiblity, which is needed to enable the size to be applied to the paper in an aqueous vehicle, the most practical way. Of course, hard water causes these salts to precipitate out of solution, which lowers their sizing efficiency and causes deposit problems on paper machines.
The most efficient oil-sizing agents for cellulosic materials contain long, linear fluorocarbon chains. Fluorocarbons, in general, have very low surface energies and are not wet easily by oil based materials, but salts that contain fluorocarbon hydrophobes are inefficient water sizing agents. Some agents that contain fluorocarbon hydrophobes but no hydrophilic salt groups do provide both water and oil sizing, but their level of water sizing is very low. Conversely, chemicals that contain hydrocarbon hydrophobes are efficient water sizing agents, but are inefficient oil sizing agents. Consequently, in a combination of a fluorocarbon based product for oil resistance and a hydrocarbon-based product for water resistance, each product has an adverse effect on the other's performance.
For instance, the inclusion of a hydrocarbon based size such as an alkyl ketene dimer (AKD) decreases the ability of a fluorochemical size to resist oil and grease penetration and requires more fluorochemical to attain the required level of oil resistance.
Ketene dimers, which are .beta.-lactones of 3-hydroxy 3-butenoic acids, have the generic structure ##STR1## When R' and R"' are hydrogen, the compounds are referred to as aldoketene dimers.
The aldoketene dimer in which R, R', R", and R"' are H is the one commonly referred to as ketene dimer. Aldoketene dimers are typically produced by letting the precursor aldoketenes spontaneously dimerize, as follows: EQU 2 RHC.dbd.C.dbd.O.fwdarw.Aldoketene dimer
The preparation of the aldoketenes is well known, for instance from S. Patai, "The Chemistry or Ketenes, Allenes, and Related Compounds", J. Wiley and Sons 1980., which discloses that when R is a perfluoroalkyl group (R.sub.f), no aldoketene dimers are formed: EQU 2R.sub.2 HC.dbd.C.dbd.O.fwdarw.No Aldoketene dimer
In the alkyl ketene dimer (AKD) that is sold commercially as a sizing agent for cellulosic materials, R and R" are long chain alkyl groups. Other variations of R and R" include R groups that contain alkyl, ester, ketone, ether, isocyanate, aryl, chloro, bromo, and heptafluoroisopropoxy functionalities.
U.S. Pat. No. 3,795,684 discloses a heptafluoroisopropoxy group on a .beta.-lactone, but fails to suggest that its sizing ability extends beyond "water repellency" which follows from the fact that the fluorinated segment of the molecule is very short and branched.
U.S. Pat. No. 3,362,965 discloses "mixed" aldoketene-ketoketene .beta.-lactones produced by the reaction of aldoketenes, RHC.dbd.C.dbd.O, with perfluorinated ketoketenes, (R.sub.f).sub.2 C.dbd.C.dbd.O, without any suggestion of sizing ability other than water repellency.
There is a clear need for an efficient size such as an alkyl ketene dimer that has the ability of a fluorochemical size to resist oil and grease penetration without the inhibiting effect of a separately added fluorochemical.